The present invention relates to an improved medical ultrasound Doppler unit of the type used for measuring velocity of blood flow.
Pulsed Doppler velocimeters can discern only a limited range of Doppler-shifted frequencies. This limitation arises from insufficient time-sampling of the Doppler signal. Pulsed Doppler velocimeters sample a Doppler-shifted signal at a single arbitrary depth, a depth determined by the delay between the insonifying pulse and the sampling time. This sampling, performed at a rate called the Pulse Repetition Frequency (PRF), limits the maximum unambiguously discernable Doppler-shifted frequency, and, therefore, the maximum discernable velocity.
The Nyquist Sampling Theorem implies that a pulsed Doppler velocimeter can unambiguously discern only those Doppler-shifted frequencies which are between -PRF/2 and +PRF/2. Any Doppler-shifted frequency outside of this interval (hereafter called the "Nyquist interval") will be aliased, that is, it will appear to be at a frequency that is inside this interval. Without additional information, the pulsed Doppler velocimeter cannot discern whether a perceived Doppler-shifted frequency is actually within the Nyquist interval or whether it is an alias of a frequency outside of this interval.
Mathematically, the perceived frequency, f.sub.p, of a signal sampled at f.sub.s and having a true frequency, f.sub.t, is found by: EQU f.sub.p =f.sub.t -f.sub.s *ROUND(f.sub.t /f.sub.s) (eq 1)
where ROUND(X) is a function which rounds the number inside the parentheses, i.e., ROUND(X) will be an integer which is equal to the greatest integer in X, plus 1 if the remaining fractional part is greater than or equal to 0.5.
For pulsed Doppler velocimeters, this equation is written as: EQU f.sub.p =f.sub.6 -PRF*ROUND(f.sub.t /PRF) (eq 2)
Thus, the perceived frequency is always within the Nyquist interval. If the true frequency is also between -PRF/2 and PRF/2, then no aliasing occurs, and the perceived frequency is the true frequency. If the true frequency is outside the Nyquist interval, then the true frequency appears to be the perceived frequency as found in (eq 2). The ROUND function in the above equation can alias several possible true frequencies (f.sub.t 's) onto the same perceived frequency f.sub.p. Accordingly, there is no direct way to determine whether the perceived frequency is the true frequency of one of many possible aliases.
Methods used heretofore to circumvent the aliasing problem included using continuous wave (CW) Doppler, a common technique. However, continuous wave Doppler loses all range resolution.
Another approach is to decrease the transmitted frequency in order to proportionally decrease the Doppler-shifted frequency. A disadvantage of this common technique is the problem of decreased scattering and decreased partial resolution.
Increasing the PRF in order to increase the Nyquist interval is also a commonly used technique However, it is subject to range ambiguities. The signal from a desired range cell of depth, d, is sampled by delaying the sample with respect to the insonifying pulse by a time, t, found by: EQU t=2d/c, (eq 3)
Where c is the propagation velocity of the insonifying signal. Equation 3 does not separate the desired range's signal from signals coming from deeper ranges (which were insonified by prior pulses). That is, it also receives signals from ranges at the following depths: EQU d(n)=(c/2)*(t+nT), (eq 4)
Where T is the pulse repetition period (=1/PRF), and n is an integer which is greater than or equal to 1.
This interference is not a problem when T is sufficiently large (the PRF is sufficiently low). In such case, the unwanted range cells are so deep that their signals are sufficiently attenuated by the propagating medium and, therefore, are so weak that they do not interfere with the desired signal. However, as the PRF increases, the unwanted ranges move closer to the insonifying source, and their signals become strong enough to interfere with the desired signal.
This interference has been exploited to advantage by the high-PRF or "extended range" concept, whereby the desired range is not the closest received range but rather one of the deeper ones. This technique assumes, however, that the signals coming from the shallower, unwanted ranges are negligible.
The main disadvantage of the extended-range concept is that we do not know that the interference from undesired ranges are negligible.
Another approach which was used heretofore involves estimating the Doppler frequency f(i) at a time t(i), and assuming that estimates at times t(j), near t(i), are also close to (within .+-.PRF/2 of) this estimate. If a perceived frequency, f.sub.p (j), is outside this range, it is replaced with the "most-likely" true frequency, f.sub.ml (j). f.sub.ml (j) is chosen from all the possible true frequencies which alias to the perceived frequency f.sub.p (j). The one chosen is the one closest to f(i) and is found by the following formula: EQU f.sub.ml (j)=f.sub.p (j)+PRF*ROUND((f.sub.ml (i)-f.sub.p (j))/PRF) (eq 5)
The baseline estimate, f(i), is periodically updated in order to track non-stationary Doppler spectra (such as caused by the variation of blood velocity distribution over the cardiac cycle, as seen by Doppler blood velocimeters).
One disadvantage of this approach is that the original baseline estimate must not be aliased, or else the future estimates will be corrupted.
A second disadvantage of this method is that the technique assumes that the time between frequency estimates is sufficiently short such that the difference between the true frequencies f.sub.t (j) and f.sub.t (i) is within the Nyquist interval. If the Doppler spectrum changes sufficiently between updates, then future estimates will be corrupted.